1. Field of the Invention
The present invention relates generally to a hot-wire air flowmeter control system for an internal combustion engine, and more particularly to a system for controlling a hot-wire air flowmeter including a hot wire provided for an internal combustion engine having a canister for storing and supplying fuel evaporated from a fuel tank into an intake passage according to engine operating conditions. The hot wire is disposed within an intake passage to measure the amount of air when cooled, and heated up at appropriate timings to burn out contamination sticked thereonto.
2. Description of the Prior Art
Conventionally, system for controlling a hot-wire air flowmeter provided for an internal combustion engine are well known. For instance, one of these systems is disclosed in Japanese Published Unexamined Patent Application No. S56-146022 for HOSOBUCHI, Electronically-Controlled Fuel Injection System.
The hot-wire is usually disposed within an intake passage in order to measure the amount of air flowing through the intake passage on the basis of the amount of heat radiated from the hot wire when cooled by the flowing intake air. The amount of fuel to be injected into an internal combustion engine is determined on the basis of the amount of intake air measured by the hot-wire air flowmeter. By the way, the hot wire is often heated up to about 1000.degree. C. for a fixed time period (e.g. 1 to 2 seconds) by passing a large current therethrough in order to burn out contaminations sticked onto the hot wire. In heating up the hot wire, since the hot wire is cooled when the intake air is flowing through the intake passage and therefore the burning temperature of the hot wire is reduced, the hot wire must be heated up when the speed of intake air is zero, that is, when the engine is stopped perfectly. To determine the state where the engine is kept stopped, the condition that an ignition switch is turned off is usually detected and a large current is passed through the hot wire when a predetermined time period (e.g. 5 seconds) has elapsed after the ignition switch is turned off.
On the other hand, a canister is often provided for an internal combustion engine for storing fuel evaporated from a fuel tank and for supplying or restoring the fuel stored therein into the intake passage according to the engine operating conditions. To supply the fuel stored in the canister to the intake passage, a purged air control valve is provided for the canister. This control valve is opened or closed in dependence upon the difference in pressure between the upper stream side and the lower stream side of a throttle valve arranged within the intake passage.
In an internal combustion engine provided with both the hot-wire air flowmeter and the canister, in case fuel stored within the canister is excessively supplied into the intake passage due to canister trouble such that the purged air control valve malfunctions and therefore the engine stops, there exists a problem in that the rich mixture is fired within the intake pasage when the hot wire is heated for contamination removal after the ignition switch has been turned off.
A more detailed description of the prior-art hot-wire air flowmeter control system for an internal combustion engine provided with a canister will be made hereinafter with reference to the attached drawing under DESCRIPTION OF THE PREFERRED EMBODIMENTS.